Light-Emitting Diodes (LEDs) are proven reliable light sources in fiber optic communications systesm. LEDs have substantially longer operating lifetimes than lasers. Conversely, lasers emit more optical power and are capable of higher modulation rates. The slower switching speeds of present high-radiance LEDs restrict these devices to applications requiring low to moderate data rates in the order of 50 Mbits. The LED modulation bandwidth determines the maximum achievable data rate. The modulation bandwidth is inversely related to optical power, and controlled by the doping density of the semiconductor material. In the fabrication process, optical power is often traded for speed to obtain high-speed devices for high bit-rate LED based communication systems.
It is a primary objective of the present invention to achieve higher data rates from the same LED(s) without sacrificing optical power.
A related object is to enhance the optical switching times of an LED and thereby effectively increase the LED speed-power product.
An LED driver circuit in accordance with the present invention enhances optical switching times by conditioning the drive pulse to the LED. The driver has fast turn-on and turn-off capabilities enabling an LED to be operated at high data ratrs (e.g.,&gt;300 Mbits). A current regulator is series coupled with the LED and serves to maintain a constant current to the LED over time, temperature, and supply voltage variations. During turn-off, this current is shunted away from the LED by a low impedance emitter follower, operated in the active non-saturated region, and a small reverse bias potential is momentarily supplied to the LED which quickly sweeps out the charge stored in the LED junction. At the end of the "off" cycle there is a forward prebias potential across the LED. During turn-on, the current shunt is reverse biased and a current peaking circuit provides a momentary additional current through the LED. This instantaneous forward current peaking and the forward prebias both serve to substantially reduce the turn-on delay and rise time of the optical drive signal. The current peaking and current shunting circuits are driven from the same differential amplifier, which minimizes timing errors. Also, by driving both LED turn-on and turn-off circuits from one source, improper sequencing of current peaking and current shunting resulting from variations in propagation delays is minimized.